$O(N)$ Models with Boundary Interactions and their Long Range Generalizations

TL;DR

This paper investigates boundary-localized $O(N)$ models in various dimensions, revealing non-trivial boundary conformal field theories with conserved currents and exploring their long-range generalizations, including critical exponent estimates.

Contribution

It introduces new boundary $O(N)$ models with boundary interactions, analyzes their critical properties using large $N$ and epsilon expansions, and connects them to long-range models with non-local descriptions.

Findings

Existence of non-trivial $O(N)$ boundary conformal field theories in $1<d<4$.

Bulk higher-spin currents are conserved up to boundary terms.

Critical exponents estimated for long-range $O(N)$ models in $d=1$.

Abstract

We study the critical properties of scalar field theories in $d+1$ dimensions with $O(N)$ invariant interactions localized on a $d$-dimensional boundary. By a combination of large $N$ and epsilon expansions, we provide evidence for the existence of non-trivial $O(N)$ BCFTs in $1<d<4$. Due to having free fields in the bulk, these models possess bulk higher-spin currents which are conserved up to terms localized on the boundary. We suggest that this should lead to a set of protected spinning operators on the boundary, and give evidence that their anomalous dimensions vanish. We also discuss the closely related long-range $O(N)$ models in $d$ dimensions, and in particular study a weakly coupled description of the $d=1$ long range $O(N)$ model near the upper critical value of the long range parameter, which is given in terms of a non-local non-linear sigma model. By combining the known perturbative descriptions, we provide some estimates of critical exponents in $d=1$.